1. Field of the Invention
The present invention relates to an optical semiconductor device and a semiconductor laser device for frequencies within a frequency range of millimeter waves to terahertz waves (30 GHz or more to 30 THz or less). More particularly, the present invention relates to a cascade laser device which has a so-called cascade laser structure and can be used in such an application as laser oscillation, optical amplification, and photodetection.
2. Description of the Related Art
As a new type of semiconductor laser, a semiconductor laser named a quantum cascade laser and based on sub-band transition of a carrier in the same energy band in a conduction band or a valance band. The oscillation wavelength of the quantum cascade laser depends on an energy interval between two sub-bands in regard to optical transition. Thus, oscillation wavelengths may be selected from a wide spectral region (from middle infrared region to terahertz band). As disclosed in Nature. Vol. 417, 156 (2002), at first, it has been substantiated that a configuration in which an oscillation wavelength of 4.2 μm in a middle infrared region is selected enables realization of such a semiconductor laser. In a proposal of Japanese Patent Application Laid-Open No. 2000-101201 which offers a different sub-band configuration, laser oscillation at 7.2 μm in a middle infrared region is achieved. A recent demand for electromagnetic resources of a terahertz band, which is considered useful for biosensing, has led to developments of long-wavelength lasers which select oscillation wavelengths of a longer wavelength than that in a middle infrared region. Japanese Patent Application Laid-Open No. 2006-032691 discloses a laser device of about 120 μm (about 2.5 THz) in a terahertz band.
Referring to FIG. 4, a configuration of a quantum cascade laser is outlined.
FIG. 4 illustrates a part of a conduction band structure when a designed electric field is applied to the quantum cascade laser. An active region 410 includes, for example, barriers 441, 443 and 445 and quantum wells 442, 444 and 446. These components constitute sub-bands 411, 412 and 413 in the active region 410. A relaxation region 420 includes barriers 451, 453, 455 and 457 and quantum wells 452, 454, 456 and 458. These components constitute a mini-band 421 formed by bundling up multiple sub-bands. Thus, the quantum cascade laser has such a feature that a plurality of active and relaxation regions 410 and 420 are alternately repeated. An active region 430 is a next active region in the repetition.
When the designed electric field is applied to the quantum cascade laser, current flows as follows. Electrons cause optical transition 401 from the sub-band 411 to the sub-band 412 in the active region 410 to emit light of a wavelength equivalent to the energy interval between the sub-bands 411 and 412. Subsequently, the electrons of the sub-band 412 of the active region 410 pass through the sub-band 413 by optical phonon scattering 402 and the like to achieve population inversion between the sub-bands 411 and 412, and are extracted to the relaxation region 420. The electrons that have passed through the mini-band 421 of the relaxation region 420 are injected to the next active region 430 to cause the same optical transition as that of the active region 410.
As a configuration of a relaxation region in such a typical quantum cascade laser, a configuration that uses a mini-band as described above has been disclosed. The quantum cascade laser in Japanese Patent Application Laid-Open No. 2006-032691 is an example where the active region is provided with no relaxation mechanism. By using the sub-band for the relaxation region, population inversion is achieved between the two sub-bands involved in optical transition in the active region.
As described above, the conventional quantum cascade laser uses the mini-band formed by bundling multiple sub-bands of equal energy or the sub-band for the relaxation region to obtain cascade-connection of the active regions.
Under these technical circumstances, the present invention has focused on the following point. That is, presence of a sub-band or a mini-band in the active region is necessary for utilizing transition between the sub-bands. However, the presence of a sub-band or a mini-band in the relaxation region is not always necessary if the following requirements are satisfied. A first requirement is that carrier extraction/injection can selectively be carried out for a sub-band in the active region. The second requirement is that multiple cascade-connected active regions can contribute to one electromagnetic mode.